A sensor with an electrolyte-insulator-semiconductor structure (abbreviated EIS structure), comprises a semiconductor substrate on which an insulator is arranged, which, during measurement mode is supplied with an electrolyte. Ion-sensitive field effect transistors (ISFETs) are established examples of sensors with an EIS structure, wherein, in this case, the insulator forms the ion-sensitive gate insulator of a field effect transistor.
In the case of the so-called LAPS (short for Light-Addressable Potentiometric Sensors), by means of a modulated light signal, photoelectrons are produced in the semiconductor material of an EIS structure, wherein the generation of photoelectrons, again, depends on the electrolyte properties. A basic description of LAPS is given by Hafeman et al. in “Light addressable potentiometric sensor for biochemical systems”, Science 240 (1988) 1182-1185.
ISFETs are more established and better investigated than other EIS structures. Therefore, in the following description of problems in the state of the art, reference is essentially made to ISFETs, wherein it is inherent that the described problems and the solution of the invention with all described embodiments are correspondingly given for other sensors with an EIS structure.
An ISFET is an ion-sensitive field effect transistor with a semiconductor substrate, a source, a gate and a drain, which, depending on the particular embodiment of (and especially choice of material for) its gate, can, based on the conductivity of the transistor, measure an ion concentration in a measured liquid (e.g. a concentration of H+, or H3O+ ions) and therewith the pH value of the measured liquid.
Sensors with ion-sensitive field effect transistors (in the following abbreviated to ISFET sensors) are used for measuring ion concentrations or special substance concentrations in liquid measured media of different compositions and conductivities Application of ISFETs for continuous detection of concentrations occurs in environmental monitoring, in industrial process monitoring, in the foods industry and in biochemistry/medical technology. ISFET sensors are especially wide spread for determining the concentration of H+, or H3O+ ions and the pH value derived therefrom in a measured liquid. Important in the case of the applications of the ISFET sensors named above is a highly precise concentration registration and a fast start-up, while at the same time maintaining an acceptable price.
The ion concentration (or the pH value) to be determined for a measured liquid at the gate of the ISFET influences the channel resistance of the ISFET, which, for example, in the case of applying a constant voltage between the source and drain, expresses itself in a change in the channel current. Exploiting this effect, the measurement circuit of an ISFET sensor produces an output signal, which is correlated with the ion concentration or the pH value of the measured liquid. Different measuring electronics for a pH ISFET sensor are known, for example, from “Analytical and Biomedical Applications of Ion Selective Field-Effect Transistors” P. Bergveld, A. Sibbald, Elsevier Science Publishers B.V., Amsterdam 1988, Chapter 8, ISFET Instrumentation, Pgs. 101-107.
The measurement circuit can, for example, be embodied in such a manner, that the channel current between the drain and source is held constant. This can, for example, be achieved by application of a negative feedback circuit, which impresses upon the reference electrode (or, alternatively, the source-electrode) a potential required for holding the channel current constant.
The gate of an H+, or pH, sensitive ISFET includes an H+, or pH, sensitive layer (for example of A12O3, Si3N4 or Ta2O5), which, for measuring, is brought directly in contact with the measured liquid. For ISFET sensors having a Ta2O5 gate layer, upon turning on the measuring electronics, it can especially be observed that the output signal of the sensor first reaches a stable value after some minutes. Only after reaching a stable value, however, is the performing of measurements possible with sufficient accuracy.